1. Field of the Invention
This invention relates to an integrated optical semiconductor device having a semiconductor optical waveguide and a light absorption semiconductor layer integrated therein, and more particularly to an integrated optical semiconductor device having an improved interface portion between the optical waveguide and the light absorption layer.
2. Description of the Related Art
In recent years, an optical communication system using a wavelength of 1.55 .mu.m at which the fiber loss becomes minimum has been put into a practical stage, and higher operation speed and larger capacity have been required. However, the present optical communication system is a system which simply utilizes ON and OFF of light and the system has limited operation speed and capacity. For this reason, coherent optical communication systems utilizing the property of light acting as wave are studied from various points of view.
In order to put the optical communication system of high speed and large capacity into practice, it is necessary to integrate electronic devices and optical devices, and an OEIC (opto-electronic-integrated circuit) having electronic elements and optical input/output ports such as light emitting devices/photo detectors integrated therein has been studied for a long period of time. However, in the coherent optical communication system, since the property of light acting as wave is strongly exhibited, it becomes necessary to integrate an optical path such as an optical isolator and an optical waveguide in addition to the above elements, but the study of integration of the optical path and the like is still at the initial stage.
Conventionally, as an example of an integrated optical semiconductor device which has long bee studied, an integrated optical semiconductor device formed by evanescent wave coupling the optical waveguide and the photoelectric element to each other and integrating them is provided. According to the conventional device, an InGaAsp layer 2 having a band gap corresponding to a wavelength of 1.1 .mu.m is buried in an InP substrate to constitute an optical waveguide. An InGaAs light absorption layer, InAlAs Schottky barrier layer and a pair of Schottky electrodes are laminated on a portion of the optical waveguide to constitute a light absorption region.
In the conventional device with the above construction, the refractive indices of InP and InGaAsP are respectively 3.2 and 3.4, and if an interface at which the refractive index is sharply changed is present, light is reflected at the interface. Further, since the refractive index of InGaAsP is larger than that of InP, light travels inside the InGaAsP waveguide. The field strength E of light at this time exhibits such a distribution that the field is high at the center of the optical waveguide but part of the field leaks to the exterior.
If the light absorption layer exists at the foot portion of the electric field distribution, part of the light is absorbed into the light absorbing layer. For this reason, when light is traveling in the light absorption region, the electric field E is gradually decreased. If, in this case, a reverse bias voltage is applied to the optical semiconductor element through a pair of electrodes and the light absorption region is depleted, absorbed light causes electron-hole pairs which are in turn derived out from the electrode as current.
In the above integrated optical semiconductor device, some subjects which must be improved are present. First, when layers of different refractive indices are present on the optical waveguide, the effective refractive index of the optical waveguide is changed. Therefore, light which has traveled in the optical waveguide is subjected to sharp variation in the refractive index at the interface between the optical waveguide and light absorption region and consequently part of the light is reflected at the interface. This causes reduction in the absorption efficiency of the photoelectric element and an increase in noise of the laser due to the returned light.
Further, since a large portion of incident light is absorbed on the incident side of the light absorption region, electron-hole pairs are densely generated on the incident side of the light absorption region. In the case of a coherent optical receiver, local oscillation light of more than several mW is incident, and if the incident light is so intense, deterioration in the response speed due to the space-charge effect and the breakdown of the Schottky barrier due to the concentration of current may be easily caused. Reverse bias of a PN junction is frequently used as means for depleting the light absorption layer, but also in this case, the PN junction may be easily broken down.
If the absorption coefficient of the entire light absorption region is lowered in order to prevent the reflection and space-charge effect, the length of the light absorption region required for absorbing a preset amount of light becomes large, thereby increasing the element area and causing reduction in the response speed due to an increase in the capacity.
Thus, in the conventional integrated optical semiconductor device having the semiconductor optical waveguide and the light absorption semiconductor layer laminated together, the degree of reflection of incident light at the interface between the optical waveguide and the light absorption layer is large, thereby causing reduction in the absorption efficiency and an increase in noise of the laser. Further, electron-hole pairs are densely generated on the light incident side (rear side of the optical waveguide with respect to the light traveling direction) of the light absorption region, thus causing deterioration in the response speed and breakdown of the device.